Controlled radical polymerization has been widely used to synthesize different polymer architectures such as block copolymers, star polymers, gradient copolymers and polymer brushes. One of the interesting applications is the synthesis of polymer brushes on nanoparticles for incorporation into polymer nanocomposites. In polymer nanocomposite science, it has been well established in the recent years that chemical grafting of a homopolymer having the same chemistry as that of the polymer matrix onto the inorganic nanoparticle substrate shields the nanoparticle from the matrix. Theoretically this should prevent the matrix from de-wetting the substrate. However, in several instances there still remains an unfavorable entropic interaction between the grafted chains and the matrix, which causes the system to de-wet. This phenomenon is known as autophobic de-wetting and is described in detail by many researchers and is now well understood. Through simulation, Matsen et al. have demonstrated that the key to suppressing autophobic de-wetting lies at broadening the brush polymer/matrix polymer interface which can be achieved by using a bimodal system that contains a small number of long homopolymer chains in a brush which primarily consists of short dense brushes.
Although single component monodisperse polymer brushes have been successfully grafted from a variety of substrates including silica nanoparticles using a wide variety of techniques, there are surprisingly very few methods in the literature describing the synthesis of mixed polymer brush grafted surfaces. The first synthesis of mixed brushes was carried out independently by Minko et al.4 and Dyer et al.5 using the ‘grafting from’ approach based on surface anchored azo initiators. The synthesis of mixed brushes using controlled radical polymerization has been demonstrated by Zhao et al. who used a two-step ATRP and NMP reaction to graft PMMA and PS from a silica surface. However, these methods utilized a split anchoring agent (i.e., a “Y” anchoring agent with two functionalized groups extending therefrom). Then, each functionalized group can be used to attach a polymeric chain thereto.
As such, these methods are limited in the types of monomers required for separate polymerization. Additionally, these methods necessarily form a particle with a 50/50 percentage of each polymeric chain.
Thus, a need exists for improved methods for synthesizing nanoparticles with multiple polymer assemblies attached.